1. Field of the Invention
This invention relates to improvement of an electrophotographic member having a coated film formed by coating and drying a coating solution containing a photoconductive compound on a cylindrical substrate surface.
2. Description of the Prior Art
As the method for preparation of electrophotographic photosensitive members currently employed, there have been adopted the methods in which Se, Se-Te, As.sub.2 Se.sub.3, Sb.sub.2 S.sub.3, Sb.sub.2 Se.sub.3, CdS or Si is provided on electroconductive substrates in various manners such as vapor deposition, or in which coating solutions comprising dispersions of inorganic or organic photoconductive pigments or solutions of organic photoconductive compounds, which may contain binder resins if desired, are coated on electroconductive substrates, followed by the step of drying, to produce photosensitive members.
In particular, the production method according to the latter coating--drying steps, which enables continuous production, may be said to be advantageous in aspect of the preparation steps.
As the photosensitive members applicable in these preparation steps, there have been known products coated with resin dispersions of CdS, ZnO, TiO.sub.2, etc. and optionally added sensitizers, or solutions of organic photoconductive compounds such as polyvinylcarbazole containing appropriate sensitizers. Also, in recent years, as the coating type photosensitive members, there have been developed so called organic type functionally separated type photosensitive members prepared by coating and drying dispersions of phthalocyanine type compounds, perylene type compounds, azo type compounds, quinacridone type compounds or other various organic type dyes or pigments to form a charge generating layer and then coating and drying resin solutions containing compounds such as pyrazoline derivatives, hydrazone derivatives, diphenylmethane derivatives, triphenylmethane derivatives, triphenylamine derivatives, oxadiazole derivatives, benzooxazole derivatives, styryl dye base derivatives and others to form a charge transport layer; or photosensitive members prepared by coating solutions of co-crystalline complexes of dyes and resins. These photosensitive members are not only excellent in sensitivity and durability but also advantageous in production aspect such as processability, cost, etc., and therefore they find uses in various applications of which scope is still becoming wider.
However, while the coating type photosensitive members have the advantages in that they can be continuously produced, and so on, unevenness in heating in the drying step causes sensitivity irregularities or charging irregularities which are problems in characteristics of the photosensitive members, thus being the primary cause for lowering of yield. As the possible reasons why the scattering in the drying step leads to the defects in the electrophotographic characteristics, there may be considered the problem that uneven heating may result in partially differing vaporization speeds of the solvent, whereby the concentrations of the molecules or particles of photoconductive compounds or sensitizers contained in the binder resin may become ununiform or that partial difference may be formed in micro-Brownian movements of molecules or particles of the photoconductive compounds or sensitizers, to result in causing an ununiform agglomerated state. When drying is completed under the state where such a partial unevenness is created, irregularities in characteristics appear to be generated in the electrophotographic photosensitive member. In the prior art, with regard to the drying step in the preparation steps, it is practiced with elaborations on the device or with the greatest care while setting severe drying conditions. Nevertheless, under the present situation, the drying step is still a cause for generation of unacceptable products.
In particular, this tendency is further pronounced in the functionally separated type photosensitive members or the photosensitive members comprising co-crystalline complexes as mentioned above. That is, the drying step is a cause for giving rise to generation of irregularities in characteristics in the photosensitive members, since in such functionally separated type photosensitive members, the pigment particles employed for the charge generating layer are extremely minute and therefore influenced greatly by the Brownian movements to be prone to cause agglomeration, while in case of co-crystalline complexes, it is liable to have influences on thermal equilibrium in formation of complexes.
At present, concerning these coating type photosensitive members, especially the functionally separated type photosensitive members or the co-crystalline complex type photosensitive members susceptible to the problems in the drying step, as an example to countermeasure generation of drying irregularities, there is the method in which a sheet-shaped substrate is employed. A sheet-shaped substrate, which is rolled in a form of roll as the original plate, is subjected continuously to the coating step, the drying step and the cutting step to be worked into a photosensitive member. In the drying step, it is possible to use a drying furnace which can be designed so as to perform drying for a relatively long time, whereby more uniform drying can be effected by gradual heating and gradual cooling. Further, it is also possible to adopt such a constitution that a dry hot air is evenly blown against the coated surface, thus avoiding uneven heating.
On the other hand, when a sheet-shaped photosensitive member is applied to a copying machine, the copying step is performed with said member mounted on a belt-shaped or drum-shaped driving support. During this operation, due to the presence of the seam of the sheet-shaped photosensitive member, it is necessary to provide the body of copying machine with a registration mechanism during copying, and also the operations at the time of exchange of the sheet-shaped photosensitive member are complicated. Further, the sheet-shaped photosensitive member is required to have a surface area greater than the size of originals to be copied, thus involving inherently the problems in designing of a copying machine such that the body of copying machine becomes greater in size.
In view of the various points as mentioned above in designing of a copying machine, a photosensitive member may desirably of a cylindrical shape without a seam and of a type uniformly coated. However, a solution containing a photoconductive compound coated on a cylindrical substrate, unlike the aforesaid sheet-shaped substrate, can hardly be dried evenly. For example, when considering a drying machine structure in which a coated cylindrical photosensitive member is continuously moved through a drying furnace, although gradual heating and gradual cooling may be possible during the step, it is impossible to blow a dry hot air evenly at any individual portion on the surface of the photosensitive member. Further, in a device with a constitution wherein one photosensitive member is placed in one drying furnace for drying, heating may be applied evenly on the entire surface of the photosensitive member from the surrounding, but such a constitution is not practical in aspect of continuous production, because it will take a relatively longer time for drying.
In the prior art, in the production of an electrophotographic photosensitive member comprising coating and drying steps, various investigations have been made about the coating techniques and, not depending on the shape or the material of the substrate, homogeneous coated films can be obtained with a uniform thickness and without irregularity. On the other hand, the drying step in case of using a cylindrical substrate, unlike the case of using a sheet-shaped substrate, has not been investigated consistently but the drying conditions are adjusted in conformity with the manner of the step, and such a technique may be said to be one belonging to the region of know-how. Moreover, uniformity in the drying step cannot be judged to be good or bad simply by visual observation, but the product is required to be good and uniform in the potential characteristic.